1. Field of the Invention
The present invention relates to a frame rate conversion, and more particularly, to a method and apparatus for adaptive frame rate conversion.
2. Description of the Prior Art
Generally, a video signal containing a motion picture displays a certain fixed number of still pictures one after another per second instead of a continuous signal. Each displayed fixed picture is called a frame. The number of frames displaying per second is defined as a frame rate of this video signal. At present, each country and video player adopt different video signal formats, also accompanying various frame rates. Therefore, it is indispensable to have an appropriate frame rate conversion method between different video signal formats so that they are played and watched in the right visual effect. For example, once two present video signal formats, NTSC with a frame rate of 30 frames per second and PAL with a frame rate of 25 frames per second, are expected to be compatible, the frame rate conversion has to be performed. Obviously, there is the difference of 5 frames per second between the two formats. That is, 5 frames have to be removed in per second when converting NTSC to PAL, and equivalent frames have to be inserted in the reverse operation in order to equalize the frame rate. Usually, two aspects are considered to deal with frames to carry out the frame rate conversion. One is dealing with an entire frame and the other is for parts of a frame. These two aspects are described as following respectively.
For dealing with an entire frame in frame rate conversion, there are frames that have to be removed or inserted to equalize the frame rate between different video signal formats. Consider conversions shown in FIG. 1A and FIG. 1B, schematic diagrams of the aforementioned example about NTSC format converting to PAL format, and vice versa. F00 to F11 . . . represent the frames to be played per second in the two video signals, wherein only partial frames per second are drawn in order to make the diagrams concise and to provide a clear description for an easy understanding. In FIG. 1A accordingly, one frame has to be removed in a 6 frame interval for NTSC, thus 5 frames (such as F05, F11, etc.) are removed totally per second and the frame rate of 25 frames per second is achieved that is equal to the frame rate of PAL, hence the conversion from NTSC to PAL is then accomplished. In FIG. 1B, a similar mechanism with FIG. 1A but operates reversely, one frame has to be inserted in a 5 frame interval, thus 5 frames (such as F041, F091, . . . etc.) are inserted totally per second and the frame rate of 30 frames per second is achieved that is equal to the frame rate of NTSC, hence the conversion from PAL to NTSC is then accomplished. While the conversion is completed via removing frames, we say that the frames are dropped and the conversion is called frame drop conversion. By contrast, while the frames are inserted to complete the conversion, we say that frames are duplicated and the conversion is called frame duplication conversion.
However, significant jump or delay phenomena of the video signal can be observed when the aforementioned frame drop or duplication conversion is utilized to convert the frame rate between different video signal formats. Consider continuous frames representing a moving object A in a video signal as shown in FIG. 1C, FIG. 1D and FIG. 1E. FIG. 1C shows the object ‘A’ moving from left to right. While the frame F02 is dropped as performing the frame drop conversion, as shown in FIG. 1D, the object A is observed appearing at left then following at right instead of moving smoothly, thus the jump phenomenon is caused. While the frame F02 is duplicated to generate frame F021 as performing the frame duplication conversion, as shown in FIG. 1E, the object A is observed staying at an identical position for a moment, thus the delay phenomenon is caused. The jump or delay phenomena are due to the aforementioned basic principle of the video signal containing a motion picture that displays a certain fixed number of still frames one after another per second. If any frames are dropped, the time vacancy is increased between the adjacent frames of the dropped frame, thus the displaying time between the adjacent frames is doubled and the jump phenomenon occurs. If any frames are duplicated, the identical frames are displayed twice hence the delay phenomenon occurs. Accordingly, the continuity of this video signal containing motion picture is also decreased.
Instead of dropping or duplicating an entire frame, it is beneficial to deal with a portion of the frame that is called a field. A frame has two fields those are top and bottom fields. Dropping one field between two adjacent frames carries out field drop conversion and the remaining fields in the two frames are combined to become a new frame. The aforementioned jump problem can be solved by applying a field drop conversion since the displaying time of a field is one half of a frame, and the time vacancy is hence reduced to one half as dropping a field instead of a frame. As performing field duplication conversion, the time addition only increases one half in each frame hence the delay problem is improved. FIG. 2A and FIG. 2B are the schematic diagrams about frame rate conversion from NTSC to PAL and vice versa with field drop or duplication conversion. F00 to F11 . . . represent the frames to be played per second in the two video signals, wherein T00 to T11 are the top fields and B00 to B11 are the bottom fields of these frames. In FIG. 2A, B02, B03, T08 and T09 are dropped, and then T02, T03, B08 and B09 are respectively combined to generate new frame F021 and F081. This is called field drop conversion, wherein selecting T03 and B08 but not B03 and T08 to form the new frame F021 and F081 is based on the consideration of well mixed selection. In FIG. 2B, T02 and B02 are duplicated to generate B021 and T021, and then combining T02 and B021 to form a new frame F021, combining T021 and B02 to form a new frame F022. This is called field duplication conversion.
Although jump or delay problems can be solved via the field drop or field duplication conversion, the drawback is its complex processes. In other hand, there is no obvious improvement as applying on vide signals containing many still images. There are no differences in dealing with frames or fields for still images according to the identical images in all frames in video signal. Frames with identical images at the same position are displayed one after another as time goes by, so no jump or delay problems will be observed even existing removing or duplicating frames.
Accordingly, the two aforementioned frame rate conversions exist corresponding advantages and drawbacks. Frame drop or duplication (also referred to frame dropped/duplicated) conversion should be applied on video signal for displaying a still image in order to enhance the speed and to simplify the processes in conversion. Field drop or duplication (also referred to field dropped/duplicated) conversion should be applied on video signal for displaying a moving image to enhance the continuity and to reduce the jump or delay phenomena as the conversion being carried out. Hence, how to take the advantage from combining the two frame rate conversions to different scenarios is the intention of this invention.